The present invention relates to the field of making polishing pads, and more specifically to providing macrotextured surfaces on polishing pads used in the chemical-mechanical planarization (CMP) of semiconductor substrates.
Chemical-mechanical polishing has been used for many years as a technique for polishing optical lenses and semiconductor wafers. More recently, chemical-mechanical polishing has been developed as a means for planarizing intermetal dielectric layers of silicon dioxide and for removing portions of conductive layers within integrated circuit devices as they are fabricated on various substrates. For example, a silicon dioxide layer may cover a metal interconnect conformably such that the upper surface of the silicon dioxide layer is characterized by a series of non-planar steps corresponding in height and width to the underlying metal interconnects.
The step height variations in the upper surface of the intermetal dielectric layer have several undesirable characteristics. Such non-planar dielectric surfaces may interfere with the optical resolution of subsequent photolithographic processing steps, making it extremely difficult to print high resolution lines. Another problem involves the step created in the coverage of a second metal layer over the intermetal dielectric layer. If the step height is relatively large, the metal coverage may be incomplete such that open circuits may be formed in the second metal layer.
To combat these problems, various techniques have been developed to planarize the upper surface of the intermetal dielectric layer. One such approach is to employ abrasive polishing to remove the protruding steps along the upper surface of the dielectric layer. According to this method, a silicon substrate wafer is mounted face down beneath a carrier and pressed between the carrier and a table or platen covered with a polishing pad that is continuously coated with a slurried abrasive material.
Means are also provided for depositing the abrasive slurry on the upper surface of the pad and for forcibly pressing the substrate wafer against the polishing pad, such that movement of the platen and the substrate wafer relative to each other in the presence of the slurry results in planarization of the contacted face of the wafer. Both the wafer and the table may be rotated relative to each other to rub away the protruding steps. This abrasive polishing process is continued until the upper surface of the dielectric layer is substantially flat.
Polishing pads may be made of a uniform material such as polyurethane or nonwoven fibers impregnated with a synthetic resin binder, or may be formed from multilayer laminations having non-uniform physical properties throughout the thickness of the pad. Polyurethane polishing pads are typically formed by placing a reactive composition in a mold, curing the composition to form the pad material, and then die cutting the pad material into the desired size and shape. The reagents that form the polyurethane or the resin binder also may be reacted within a cylindrical container. After forming, a cylindrically shaped piece of pad material is cut into slices that are subsequently used as the polishing pad. A typical laminated pad may have a plurality of layers, such as a spongy and resilient microporous polyurethane layer laminated onto a firm but resilient supporting layer comprising a porous polyester felt with a polyurethane binder. Polishing pads typically may have a thickness in the range of 50-80 mils, preferably about 55 mils, and a diameter in the range of 10 to 36 inches, such as about 22.5 inches.
Polishing pads also may have macrotextured work surfaces made by surface machining using various techniques, many of which are expensive and produce undesirable surface features of widely varying depths. Surface features include waves, holes, creases, ridges, slits, depressions, protrusions, gaps, and recesses. Some other factors which influence the macroscopic surface texture of a polishing pad are the size, shape, and distribution frequency or spacing of the surface features. Polishing pads typically may also have microtextured surfaces cause by a microscopic bulk texture of the pad resulting from factors intrinsic to the manufacturing process. Since polishing does not normally occur across the entire pad surface, any microtexture of the pad and the macrotextures made by surface machining, may only be formed into the portion of the pad over which polishing is to take place.
During the polishing process, the material removed from the wafer surface and the abrasive, such as silica, in the slurry tend to become compacted and embedded in the recesses, pores, and other free spaces within the microscopic and macroscopic bulk texture of the polishing pad at and near its surface. One factor in achieving and maintaining a high and stable polishing rate is providing and maintaining the pad surface in a clean condition. Another factor is reducing or preventing a hydroplaning effect caused by the buildup of a layer of water between the abutting surfaces of the pad and the wafer. It has also been determined that increasing the flexibility of the pad in a controlled manner will increase polishing uniformity, i.e., the uniformity of the polished wafer surface.
Thus, consistently achieving uniform and high quality polishing of wafer surfaces by conventional pads has presented three problems. The first of these is the buildup of abrasive particles and debris between the pad and the wafer causing uneven polishing and damage to both the pad and the wafer. Secondly, uneven polishing due to hydroplaning between the wafer and the pad during conventional processes has resulted in the relatively high loss of product yield due to the resulting wafer damage. Thirdly, uneven polishing and wafer damage has also resulted from overly rigid pads produced by prior art manufacturing techniques. Therefore, there is a need for a method and apparatus for providing polishing pads capable of consistently producing high quality wafers with uniformly polished surfaces.
The present invention, therefore, provides a pad grooving method and apparatus for producing a polishing pad that is capable of consistently forming uniformly polished surfaces on high quality wafers. The apparatus comprises a platen with positioning post for holding a polishing pad in position for engagement by a router to machine grooves in the working surface of the pad. In order to precisely control the depth of the grooves as they are routed in the pad, a spacing mechanism provides a constant and precise separation between the working surface of the pad and the chuck for holding and rotating the router.
The pad is placed on the supporting surface of the platen with its working surface in spaced relation opposite to the router bit. The router chuck and drive motor are supported opposite to the pad by a frame. The spacing mechanism comprises at least one, preferably two or more, stop members mounted on the frame adjacent to an aperture through which passes the router bit. An outer end portion of the bit projects beyond the stop member(s), which preferably are pins threaded within the frame so as to be axially adjustable. A vacuum system is provided for applying a vacuum to the working surface of the pad to pull the pad first against the outer end of the router bit and then against the stop member(s).
Rotation of the router bit by the motor while the vacuum is applied to the pad causes the outer end portion of the bit to cut an initial recess (hole) into the pad to a depth below its working surface. The recess depth is precisely limited by the stop member(s), which comes into contact with the working surface of the pad as the rotating bit cuts into the pad to form the initial recess. After formation of the initial recess, a lateral motion mechanism causes relative lateral movement between the rotating router bit and the pad while the vacuum maintains the pad in contact with the stop member(s).
This lateral movement causes the rotating bit to cut a groove in the pad extending away from the initial recess and having a depth substantially the same as the initial recess depth. The lateral motion mechanism may comprise upper and lower plates suspended from an overhead beam and arranged for relative movement in the x-y plane. For example, the upper plate may be mounted on the overhead beam and driven in the X-direction (along the X-axis) by one or more motorized screws; and the router frame suspended from the lower plate which, in turn, is mounted on the upper plate and driven in the Y-direction by one or more motorized screws. As an alternative, the platen may be similarly mounted for such x-y movement instead of the router frame, or both the platen and router frame may be mounted for such movement In addition, the platen may be rotated by a drive motor to provide an additional means for causing lateral movement between the router bit and the pad.
It follows from the foregoing that relative movement between the stop member(s) and the pad in the Z-direction (along Z-axis) may be provided by the vacuum as it pulls the pad toward the router bit and the stop member(s). Where the polishing pad is flexible due to its large diameter and small thickness, there may be no need to guide this pad movement. Furthermore, significant pad movement along the Z-axis may be avoided by instead moving the router bit along the Z-axis, and then using the vacuum to maintain the bit depth during lateral movement between the bit and pad.
However motion of the pad along the Z-axis may be guided by a plurality, preferably two or more, posts projecting outward from the platen along axes parallel to the rotational axis of the router bit. These guideposts also may secure the pad for rotation when the platen is rotated by a platen drive motor, and are particularly useful for grooving disks other than polishing pads, such as rigid disks of greater thickness and smaller diameter. As already indicated, the upper and lower lateral motion plates provide for lateral movement of the router bit relative to the pad along the X-axis and along the Y-axis. Therefore, the router bit may be moved relative to the pad in accordance with the Cartesian coordinates x, y and z, or in accordance with the cylindrical coordinates R, xcex8 and Z.
The foregoing relative lateral movements permit the grooves cut in the working surface of the pad to have either left or right spiral patterns, zigzag patterns with different groove densities, each following a constant radius around the pad at different radii, inner and outer circle grooves with spiral grooves or zigzag in areas therebetween, inner and outer sectors at different radii and having different spiral or zigzag patterns, or any combinations of these and other patterns. In addition, the patterned portions of the working surface of the pad may be confined only to those areas over which polishing of a wafer is to take place.
The depth of the grooves may also be varied for different patterns by axially adjusting the projecting length of the stop members, which are preferably symmetrical pins, or by axially adjusting the projecting length of the router bit relative to axially fixed stop members. To provide pads of increased flexibility, the grooves may penetrate into the pad for a depth up to 80% of the pad thickness. Pad flexibility may also be adjusted by the overall number of grooves provided, such as, for example, a pattern of 8, 32, or 64 spirals.
Grooves in the working surface of a CMP pad made according to the invention significantly reduce the hydroplaning effect during wafer polishing and, as a result, a much higher polishing rate can be achieved. A pattern with a higher number of spiral grooves can reduce the hydroplaning effect more efficiently than a pattern with a lower number of spiral grooves because more grooves will pass across the wafer surface being polished in the same period of time. An increase in pad flexibility due to the groove pattern selected may also help improve the polishing uniformity of the wafer surface. The groove density of zigzag groove patterns also may be varied to control the polishing rate distribution within different segments of the polishing pad surface and this may also improve polishing uniformity within the wafer surface.
The polishing pad provided by the present invention is ideal for polishing wafers of dielectric materials such as silicon dioxide, diamond-like carbon (DLC), spin-on-glass (SOG), polysilicon, and silicon nitride. The polishing pads also may be used to polish other wafers or disks such as those made of copper, aluminum, tungsten, and alloys of these and other metals.